The present invention relates to an apparatus for carrying out inspection or analysis of a material by using an optical means. More particularly, the present invention pertains to an apparatus which enables in situ measurement of etching pits during a dry etching process for manufacturing semiconductor devices.
Semiconductor devices are now widely employed in electronic apparatuses for improving controllability in general existing techniques. In order to store a large amount of data and to process data at high speed, semiconductor devices are demanded to increase in the scale of integration and become finer or more minute. In such circumstances, the changeover from the conventional planar element structure to a three-dimensional element structure is one of the technical matters which need to be solved.
To this end, it is necessary to form pits or grooves with a width of 0.5 to 2.0 .mu.m and a depth of 1.0 to 10 .mu.m in the surface of a semiconductor substrate. Since it is impossible to form such pits or grooves using conventional chemical solutions, it may be necessary to employ a dry etching technique using a gas which is reactive to the semiconductor substrate. However, it is difficult to set up the dry etching condition. And it is necessary to effect in-process monitoring of the etching. More practically, it is necessary to monitor in situ the etching of deep grooves or deep pits.
There is known literature describing optical monitoring of the etching of a semiconductor substrate, titled "Optical Monitoring of the Etching of SiO.sub.2 and Si.sub.3 N.sub.4 on si by the Use of Grating Test Patterns", H. P. Kleinknecht & H. Meier, the May 1973 issue of Solid State Science & Technology.
In this method, the beam of a He-Ne laser is aimed at a test pattern containing one or more diffraction gratings prepared in advance, and changes in the reflected first-order diffraction intensities are monitored, thereby allowing the etch rate and the etch depth to be recorded. This method suffers, however, from the following problem. As the width of a pattern to be measured decreases, the change in the diffraction interference intensity becomes small, so that it becomes impossible to effect satisfactory monitoring. It may be considered that such problem occurs owing to the fact described below. As the pattern width decreases, the size of the He-Ne laser beam (wavelength: about 0.6 .mu.m) and the size of a groove or pit which is irradiated with the laser beam become substantially equal to each other, and the intensity of the reflected light itself consequently becomes weak in the macroscopic observation. There is another conventional method in which a test pattern for monitoring the etching is formed on an object of irradiation in advance, and the etching of the test pattern is regarded as identical with the actual etching on the semiconductor device. This method cannot, however, satisfactorily cope with the demand for a higher degree of accuracy which arises from the increase in scale of integration of semiconductor devices. This is because the etching rate strongly depends on the etching dimensions.